Such dam structures are used to close off underground passages so that they can be used for storage. In particular these dams are used in order to prevent water from entering underground burial sites.
As a rule such dams are massive concrete structures which are provided with reinforcement and which are used along with seals to provide a durable and safe sealing of underground passages, acting in effect like a massive plug. The are described in Querschnitsabdichtungen untertagiger Hohlraume durch Damme und Propfen (P. Sitz, VEB Deutscher Verlag fur Grundstoffindustrie, Liepzig, 1982). Dams having a long life and providing a good cross-section seal are constituted as cup-shaped multiply ridged, that is multiply frustoconical, dams. Such a multiply ridged dam is for example formed with a three-part corrugation or ridge system. The three ridges have flank angles which decrease from the pressure side so that the main tension (pressure) vector is perpendicular to the fracture surfaces. The impinging tensions are taken up by an additional transverse reinforcement of the air-side or inside of the ridges.
Normally the height of the ridges is between 0.8r and 1.0r (r being the passage radius). The main disadvantage of such dams is that as a result of the liquid pressure in a large dam area there are tensions which increase with the modulus of elasticity of th surrounding rock or earth. As the dams get bigger these tensions decrease and asymptotically approach a final level. It is possible to form the multiply ridged or prismatoid dam out of four frustoconical parts. As a result of the liquid pressure there are tensions parallel to the dam which once again are present in locations of low modulus of elasticity and which must be taken up by a reinforcement which is normally constituted of steel reinforcing rings and yokes. Although with such a dam structure the maximum tensions and the passage subjected to tension are smaller than with the multiply ridged dams, it is still necessary to provide some means to maintain the dam capacity and the seal in every case.
Calculating the size of the dams relative to the earth load has hitherto been a fairly haphazard process mainly involving guesswork that only poorly emulated real conditions. New detailed research regarding underground dams of standard geometry multiply ridged, multiply frustoconical, or prismatoidal--is described in Gebirgsmechanische Untersuchungen zur Standischerheit eines Dammbauwerkes (BGR, Hannover, May 1985). This research has shown that as a result of the earth load there are also axial tensions in the dam structure which primarily are manifested as bending in the regions of smallest diameter. In addition tensions can result from nonuniform earth loads between the ends and the middle of the support.
The problem of reducing the axial tensions created by the earth pressure is particularly important in cases where the dams are used in regions where the earth is very rheologically inelastic in dependance on time, as for example in stone salt domes where there are relatively minor end loads. These disadvantageous tensions are taken up within certain limits by reinforcement of the dam structure. This necessitates a substantial extra expense for material and time. Hitherto the problem of axial tensions as a result of earth load is not fully accounted for because it was only possible to guess at the actual effects of the earth load or these loads were calculated under ideeal circumstances not strictly corresponding to actual circumstances.